1. Field of the Invention
This invention relates to a video tape recorder having rotary magnetic heads, and particularly to the type of recorder known as a time lapse video tape recorder that may be energized by a continuous series of video signals but is controlled to record, normally, only selected, time-spaced video signals. The invention is especially directed to a system that provides for playback of the recorded signals, usually in the next equal time interval after such recording takes place, to determine whether or not the recording apparatus is operating properly.
2. The Prior Art
The purpose of a time lapse video tape recorder (hereinafter referred to as a time lapse VTR) is to record only the relatively small number of signals corresponding to video images, even though such signals are applied to the apparatus on a continuous basis. Time lapse VTRs are especially useful for surveillance of banks, department stores, and other places in which a high percentage of the television images that might be recorded is of no use. In order to eliminate unnecessary use of video tape in such VTRs, the continuous video signal is gated so that only an occasional frame or field interval is recorded. The tape can be run at a lower speed corresponding to the intermittency of the signal to be recorded. For example, if only 1/60 of the fields are to be recorded, the tape may be run at 1/60 of the normal speed, without causing the tracks on which the signals are recorded to overlap each other so as to obliterate the recorded signals. However, in such apparatus the rotary magnetic head usually runs at the same speed as in a normal tape recorder and thus each incremental area of the tape is subjected to a high rate of wear due to the repetitive, frictional, scanning of the tape by the transducers. As a result, the magnetic coating material is likely to wear off, thereby making it impossible to produce a proper recording, and the air gaps in the magnetic transducers are likely to be blocked, thereby further interfering with the ability of the apparatus to record images properly or, indeed, to record them at all. Furthermore, such apparatus is likely to be operated continuously for a long period of time, for example 24 hours or even much longer, without being checked to see that the images are satisfactory. In any of these cases, when the rare occasion makes it absolutely essential to produce a properly recorded image, the apparatus may be unable to do so.